1. Field of the Invention
The present invention relates to a pre-amplifier in the field of high speed and low impedance transmission, more particularly, when low impedance transmission line is used as the transmission medium.
2. Brief Description of Related Arts
Currently, in the field of high speed and low impedance transmission, it is a conventional way to use a high speed operational amplifier as a pre-amplifier, to obtain a large bandwidth by way of higher power consumption, and accordingly, amplify the reception signals of high speed data.
Usually, existing products in current markets use a high speed amplifier for pre-amplification to strengthen reception. The high bandwidth of the operational amplifier is used to enlarge high speed input signals. However, the high speed amplifier requires higher bandwidth, which, in turn, means greater demand for power consumption.
FIG. 1 illustrates one of pre-amplifiers described above. P1, P2 represents respectively the differential pair input transistors PMOS; N1, N2, N3, N4 are load transistors NMOS with the same size; P3, P4 are load transistors PMOS with the same size; CL is the equivalent load capacitor of the output, wherein vcm is the offset voltage of Common-Mode-Feedback (CMF).
When pre-amplifying, this type of operational amplifier with the structure described above works within a saturation region. The gain with this structure is approximately as follows:
                    ⁢                                        Ad            =                        ⁢                                          (                                                      g                                          m                      ⁢                                                                                          ⁢                      p                      ⁢                                                                                          ⁢                      1                                                        /                                      g                                          mn                      ⁢                                                                                          ⁢                      1                                                                      )                            ⁢                                                g                                      mn                    ⁢                                                                                  ⁢                    3                                                  ⁡                                  (                                                            R                                              on                        ⁢                                                                                                  ⁢                        3                                                              ⁢                                                                                        R                                                  op                          ⁢                                                                                                          ⁢                          3                                                                                                            ⁢                                          1                      /                                              sC                        L                                                                              )                                                                                                      ≈                        ⁢                                          g                                  m                  ⁢                                                                          ⁢                  p                  ⁢                                                                          ⁢                  1                                            ⁡                              (                                                      R                                          on                      ⁢                                                                                          ⁢                      3                                                        ⁢                                                                                R                                              op                        ⁢                                                                                                  ⁢                        3                                                                                                  ⁢                                      1                    /                                          sC                      L                                                                      )                                                                                                    =                            ⁢                                                g                                      m                    ⁢                                                                                  ⁢                    p                    ⁢                                                                                  ⁢                    1                                                  ⁡                                  [                                                            R                                              on                        ⁢                                                                                                  ⁢                        3                                                              ⁢                                                                  R                                                  op                          ⁢                                                                                                          ⁢                          3                                                                    /                                              (                                                                              R                                                          on                              ⁢                                                                                                                          ⁢                              3                                                                                +                                                      R                                                          op                              ⁢                                                                                                                          ⁢                              3                                                                                +                                                                                    sC                              L                                                        ⁢                                                          R                                                              on                                ⁢                                                                                                                                  ⁢                                3                                                                                      ⁢                                                          R                                                              op                                ⁢                                                                                                                                  ⁢                                3                                                                                                                                    )                                                                              ]                                                      ;                                                            Its          ⁢                                          ⁢          3          ⁢                                          ⁢          dB          ⁢                                          ⁢          bandwith          ⁢                                          ⁢          is          ⁢                                          ⁢          approximately          ⁢                      :                    ⁢                                          ⁢          s          ⁢                                          ⁢          3          ⁢                                          ⁢          dB                ≈                              (                                          R                                  on                  ⁢                                                                          ⁢                  2                                            +                              R                                  op                  ⁢                                                                          ⁢                  3                                                      )                    /                      (                                          C                L                            ⁢                              R                                  on                  ⁢                                                                          ⁢                  3                                            ⁢                              R                                  op                  ⁢                                                                          ⁢                  3                                                      )                              =                        1          /                      [                                          R                                  on                  ⁢                                                                          ⁢                  3                                            ⁢                                                                R                                      op                    ⁢                                                                                  ⁢                    3                                                  )                            ⁢              CL                        ]                          =                  1          /                      (                          RoC              L                        )                                ,  
wherein Ro=Ron3∥Rop3 
It can be indicated from the above equation that when the load capacitor is fixed, the wider the 3 dB bandwidth requires, the smaller the Ro is. However, since Ro is proportional to 1/λI, wherein λ is a fixed parameter depending on the size of MOS transistor, the magnitude of the 3 dB bandwidth depends on the current I. The wider the 3 dB bandwidth is, the bigger the required current is.